A glass cactus is a semi-affectionate term used to describe someone who is beautiful, emotionally fragile, and prickly in demeanor. Often someone who fits this definition will be in need of (regardless if they say so) help or healing for emotional wounds, but as soon as you try to help, you are pricked and bleeding, again.

Example: "Starscream is a glass cactus, he's pretty, but you just can't get close to him. Anything you say can and will be used against you or, he'll lock himself in his room."